The fisher equation is
nominal interest rate = real interest rate + inflation rate.
If nominal interest rate is kept to zero, and real interest rate remains constant positive in the long term, then the inflation rate will be negative, representing deflation!
When the central bank drops nominal interest rate at 0%, the negative delta in the nominal interest rate increases money supply and increases the price level - for a period of time. The boost in demand is because demand is brought forward due to the fall in interest rates. Producers increases production to match this surge in demand, and in doing so, increasing supply of goods come into the market. If there are no more negative deltas in interest rate coming, as is the case under a zero interest rate policy where the nominal interest rate is zero bound, the increased supply is going to cause a decrease in price level due to decreased demand that was consumed to produce the earlier demand surge from the negative interest rate delta, causing deflation.
I'm sure there is a flaw to the logic above, and to the fisher equation, otherwise it would mean a ZIRP, held for a long time, would produce the deflation it was implemented to prevent! Can you point it out?